Purpose
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Explore the operation of an Arithmetic Logic Unit (ALU) |
Components used
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74181 Arithmetic Logic Unit |
Pinouts are attached.
Lab Problems
This lab focuses on the Arithmetic Logic Unit, or ALU, as described in Chapter 10 of our text.
Setup
Grab a 181 and hook it up. The ALU interface is a little more complex than we've seen:
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A0:A3 - 4-bit data input A |
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B0:B3 - 4-bit data input B |
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F0:F3 - 4 bit function output F |
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S0:S3 - 4-bit ALU select to control ALU operation |
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M - mode select input |
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Cn, Cn+4 - carry-in and carry-out bits for arithmetic operations |
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P, G - propagate-generate outputs for chaining multiple ALU's |
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A=B - comparison output bit |
To complete the lab, I recommend the following:
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Connect ALU data inputs A3-A0 to switches S3-S0 |
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Connect ALU select inputs S3-S0 to switches S7-S4 |
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Connect ALU function outputs F3-F0 to LEDs L3-L0 |
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With a scarcity of switches (as in, there aren't any left), we will hard-code a value into the B data inputs of the ALU. For this lab, B will always (unless otherwise specified) be set to 0101 (or 5). So, connect B0 and B2 to Vcc... B1 and B3 to ground. |
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Connect the carry-in bit, Cn, to logic 1. |
The ALU diagram describes the operation of the ALU in active high and active low mode. We will use active high. As a result, ALU values A, B, and F are all active high, and the carry values Cn and Cn+4 are active low.
1. Test the logic functions of the ALU
Logic functions are activated in the ALU by setting the mode bit, M, to logic 1. With mode set to 1, complete the table exercising the logic mode operations of the ALU.
Deliverable: Complete a table exercising all 16 logic functions of the ALU, for select inputs 0000 - 1111, for 3 values of A=0000, A=1111, and A=0010. The value of B in all cases will be 0101. Your table should look like:
| Select Bits | Function | A input | B input | F output |
| 0000 | A' | 0000
1111 0010 |
0101
0101 0101 |
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| 0001 | (A+B)' | 0000
1111 0010 |
0101
0101 0101 |
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| and so on... | ||||
2. Test the arithmetic functions of the ALU
Connect the mode input, M, to logic 0 to set the ALU in arithmetic mode.
Disconnect the carry-in bit, Cn, of the ALU from logic 0, and connect it to one of the pulse buttons, like PB1.
Connect the Carry out bit, Cn+4, to an LED to watch for carry-out, i.e. overflow, during your arithmetic operations.
Remember that carry-in and carry-out are active low, so you are setting and seeing the complements of these values.
Deliverable: Now, complete a similar table for all 16 arithmetic operations.
Deliverable: Test your carry-in pulse button on a number of cases and document the effect of carry-in on the function outputs.
Deliverable: Test your carry-out bit for a number of different cases, where you expect and don't expect overflow. Note these on your table.